1. Field of the Invention
The present invention relates to a low-loss, easily-manufacturable optical waveguide using a clad portion having an optical periodic structure called a photonic crystal, which is suitable for an optical waveguide-integrated optical circuit substrate designed for use in optical communications and optical data processing, and to an optical integrated circuit employing the same.
2. Description of the Related Art
A three-dimensionally shaped optical waveguide formed on a substrate has been used as an important optical element for constituting an optical integrated circuit designed for use in optical communications and optical data processing. In conventional optical waveguides, a high refractive-index portion, acting as a core portion for propagating light, is surrounded by a low refractive-index portion, acting as a clad portion. With this structure, propagating light is confined within and around the high refractive-index portion, thereby achieving wave guiding.
However, in such an optical waveguide as utilizes refractive-index difference, if curved and branching portions of the optical waveguide do not have a sufficiently gentle curve, guided light inconveniently finds its way into the low refractive-index portion, resulting in occurrence of great loss. Trying to prevent the loss, if the curved and branching portions are given a gentle curve, the substrate onto which the optical waveguide is formed needs to have an unduly large dimension, which makes it difficult to realize a compact optical circuit substrate, and leads to poor productivity.
To solve such problems, optical waveguide structures using a photonic crystal have been proposed to date (refer to Japanese Unexamined Patent Publication JP-A 8-505707 (1996), Japanese Unexamined Patent Publication JP-A2000-56146 (2000), or others). The photonic crystal is constituted by forming in an optical material a structure such that a refractive index periodically varies with cycles ranging from ca. 0.1 to 1.0 xcexcm. Such a periodic structure has a wavelength band in which no propagation of light occurs (photonic bandgap). Therefore, in an optical waveguide having such a structure that the core portion is surrounded by a photonic crystal, even in a sharp curved portion, theoretically no radiation loss occurs for guided light in the photonic bandgap.
Photonic crystal structures are roughly classified into two-dimensional and three-dimensional structures. A two-dimensional photonic crystal, when formed on a substrate, exerts a photonic bandgap effect only in a direction parallel to the substrate, and by contrast a three-dimensional photonic crystal exerts a photonic bandgap effect in every direction including directions parallel and perpendicular to the substrate.
However, a three-dimensional photonic crystal has a relatively narrow photonic bandgap and requires a complicated manufacturing process. Hence, it is to be expected that an optical waveguide which has such a structure that the core portion is surrounded by a two-dimensional photonic crystal on a substrate will be industrially applied to an optical circuit substrate or the like.
FIGS. 7A and 7B are views of a conventional optical waveguide using a photonic crystal, with FIG. 7A showing a plan view and FIG. 7B showing a sectional view. On a substrate 23 serving also as a lower clad portion is formed an optical material for constituting an optical waveguide having a core portion 21. On both sides of the core portion 21 are formed clad portions 22 so as to penetrate through part of the optical material and the substrate 23. The clad portion 22 has a periodic structure which exhibits a periodic variation in refractive index. Herein, a columnar periodic structure is employed.
However, in the optical waveguide having such a structure that the core portion 21 is surrounded by the clad portions 22 made of a two-dimensional photonic crystal, while an excellent photonic bandgap effect is exerted in a direction which is parallel to the surface of the substrate 23 and perpendicular to a light propagation direction in the core portion 21, no photonic bandgap effect is exerted in a direction perpendicular to the surface of the substrate 23. Therefore, in the direction perpendicular to the surface of the substrate 23, light is confined by exploiting the difference in refractive index between the core portion 21 and the substrate 23, or between the core portion 21 and air.
In this case, since the upper clad portion is usually constituted by air (refractive index n=1.0), sufficiently large refractive index difference can be obtained between the upper clad portion and the core portion. This substantially prevents radiation of light toward the upper portion. However, the lower clad portion is realized by using the substrate (n greater than 1.0). Structurally, it is thus difficult to obtain sufficiently large refractive index difference between the core portion 21 and the substrate 23 serving as the lower clad portion, which tends to cause radiation loss of light in the substrate 23.
Various methods have been discussed to overcome the above-stated problem. For example, there are known a construction in which a periodic structure of a thick photonic crystal pierces deeply through a lower clad portion, and a construction in which, as shown in section in FIGS. 8A and 8B, a lower clad portion is partially removed by etching or other means to form an air-bridge structure. In the optical waveguide shown in FIGS. 8A and 8B, by adopting the air-bridge structure, part of the substrate 23, which corresponds to the lower clad portion of the optical waveguide shown in FIGS. 7A and 7B, is replaced by air. This makes it possible to secure sufficiently large refractive index difference between the core portion 21 and the air constituting the lower clad portion.
However, the former construction requires a high aspect ratio, and the latter construction requires a complicated etching process and suffers from insufficient mechanical strength of the air-bridge structure. These are the problems to be solved.
The invention has been made in view of the above-stated problems with conventional art, and accordingly an object of the invention is to provide a low-loss optical waveguide having a clad portion made of a photonic crystal, which can be manufactured in a simple process.
Another object of the invention is to provide an optical circuit base component provided with said low-loss optical waveguide which has a clad portion made of a photonic crystal and can be manufactured in a simple process, said optical circuit base component enabling attainment of miniaturization and high integration.
The invention provides an optical waveguide comprising:
a substrate;
a core portion formed on the substrate; and
clad portions arranged on the substrate so that the core portion is sandwiched therebetween, the clad portions each having a periodic structure which exhibits a periodic variation in refractive index in a direction perpendicular to a light propagation direction,
wherein at least one of the periodic structures is inclined with respect to a surface of the substrate so that an interval between the periodic structures is gradually reduced toward the substrate.
According to the invention, in the optical waveguide, of the clad portions having a periodic structure and arranged on the substrate so that the core portion is sandwiched therebetween, at least one is inclined with respect to the surface of the substrate so that an interval between the clad portions is gradually reduced toward the substrate. With this inclination, a photonic bandgap effect derived from a photonic crystal having a periodic structure exerts on the below of the core portion. Thus, as compared with the conventional optical waveguide in which confinement of light is achieved by exploiting the difference in refractive index between the core portion and the substrate, said optical waveguide succeeds in confining guided light within and around the core portion more securely. Moreover, as compared with the conventional optical waveguide in which the photonic crystal is made thick, said optical waveguide has a lower aspect ratio to achieve slimness. Further, as compared with the conventional optical waveguide in which the lower clad portion is removed by etching or other means to form an air-bridge structure, said optical waveguide requires fewer manufacturing process steps and maintains adequate mechanical strength. As a result, it is possible to realize a slim, compact optical waveguide offering excellent light propagation characteristics and adequate mechanical strength, which can be fabricated in a relatively simple and easy manufacturing process.
In the invention, it is preferable that, in the optical waveguide, at least one of the periodic structures is inclined an angle of 5 to 60 degree from a direction perpendicular to the surface of the substrate.
According to the invention, at least one of the periodic structures is inclined an angle of 5 to 60 degree from a direction perpendicular to the surface of the substrate. This arrangement makes it possible to enhance the light confinement effect exerted on the core portion on the substrate side, to prevent radiation of light toward the above of the core portion, and to keep the width of the optical waveguide in an appropriate range, thereby achieving miniaturization of the optical waveguide.
In the invention, it is preferable that, in the optical waveguide, the two periodic structures make contact with each other at their lower ends.
In cases where the two periodic structures make contact with each other at their lower ends, it is possible to ensure that the photonic crystal exerts a light confinement effect on the core portion on the substrate side, there by preventing guided light from escaping from the core portion into the substrate. As a result, an optical waveguide can be realized that suffers little from loss of light and offers excellent propagation characteristics.
In the invention, it is preferable that, in the optical waveguide, the periodic structure is configured as a columnar periodic structure.
By providing the clad portion with a columnar periodic structure, for example, a triangular lattice air-rod periodic structure or a honeycome lattice dielectric columnar periodic structure, TE-mode light and TM-mode light can be concurrently subjected to a photonic bandgap effect. This helps prevent leakage of guided light having various modes and polarizing planes, thereby suppressing loss of light. As a result, an optical waveguide can be realized that is excellent in light propagation characteristics.
In the invention, it is preferable that, in the optical waveguide, a period of the periodic structure has a thickness corresponding to 20 to 60 percent of a wavelength of light propagating through the core portion.
In the invention, it is preferable that, in the optical waveguide, the periodic structure has five periods or more.
In the invention, it is preferable that, in the optical waveguide, column components of the columnar periodic structure occupy 20 to 80 percent of a sectional area of the columnar periodic structure sectioned along a direction perpendicular to a length of the column component.
The invention further provides an optical circuit base component comprising:
a substrate;
said optical waveguide formed on the substrate; and
a portion formed on the substrate, for mounting an optoelectronic conversion element which is optically coupled to said optical waveguide.
According to the invention, the optical circuit base component has the optical waveguide of the invention, and the optical waveguide is optically coupled to an optoelectronic conversion element to be mounted on the optical circuit base component. With this construction, since the low-loss optical waveguide is made smaller in size and thickness, offers excellent optical-signal transmission characteristics, and can be fabricated in a simple process, miniaturization and high integration are achieved. The optical circuit base component is accordingly suitable for an optical module or the like designed for use in optical communications and optical data processing.